1. Field of the Invention
The invention relates to sodium-sulphur cells and methods of operating them. While sodium-sulphur cells have many possible applications, they are considered particularly suitable for use in storing electrical power to provide load levelling, e.g. charging during off peak power demand periods and discharging during peak demand periods, and in electric vehicles.
2. Description of the Prior Art
Considerable efforts have been made in recent years to develope sodium-sulphur batteries, which have a high theoretical specific energy of 760 Whkg-.sup.1, though in practice at present the aim is to achieve an actual specific energy of 100 to 150 Whkg-.sup.1. A sodium-sulphur cell has advantages that it does not undergo self-discharge and is particuarly suitable for large scale energy storage.
Several sodium-sulphur cells are proposed in U.S. Pat. No. 3,951,689. One form of cell disclosed in this patent is shown in FIG. 2 of the present drawings. This cell has a ceramic solid electrolyte made of .beta."-alumina 1 enclosing liquid sodium 2 which is the anodic reactant. The anode lead 3 is inserted into the sodium 2. Outside the tubular electrolyte 1 is a cathodic reaction region 4 formed of porous graphite felt which includes a wider region at the base of the cell. The cathodic reactants are sulphur and sodium polysulphide. The sodium polysulphide is present in the felt. The sulphur 7 is kept in a separate compartment at the periphery of the cell defined by an internal wall 5 and the peripheral external wall 6. The external walls 6,10 act as the cathode. A heater indicated at 8 surrounds the cell.
During discharge operation of this cell, the sulphur store 7 is maintained at higher temperature than the cathodic reaction region 4, so that stored sulphur is vaporised and condenses in the cathodic reaction region 4 to cause a cell reaction. During charging, on the other hand, the sulphur in the store 7 is kept cooler than the cathodic reaction region 4, so that sulphur in the cathodic reaction region is vaporised and condenses in the store 7.
This cell attempts to remove the perceived defect of the prior conventional sodium sulphur cell in which both liquid sulphur and liquid polysulphide are present in the felt of the cathodic reaction region 4. This defect is that the polysulphide becomes saturated with sulphur so that the formation of further elemental sulphur in the cathodic reaction during charging limits the recharging operation. The cell of FIG. 2 removes the sulphur to a remote store.
In another embodiment illustrated in U.S. Pat. No. 3,951,689, a remote polysulphide store is provided, from which liquid polysulfide travels to the cathodic reaction means by the wicking action of a felt.
The cell of U.S. Pat. No. 3,951,689 suffers from the defect that temperature differences must be maintained between the sulphur storage region and the cell reaction region, and this temperature difference must be varied according to the operation of the cell. This is complex and cumbersome.
Furthermore, both the cell of U.S. Pat. No. 3,951,689 and the prior conventional cell described above have the disadvantage that if the ceramic electrolyte is broken, reactants of the cell immediately mix and instantaneous reaction takes place, resulting in catastrophic destruction of the cell. A second disadvantage is that the cell capacity depends upon the thickness of the felt region storing at least the polysulphide of the cathodic reactants. If the thickness of this region is increased, the cell resistance also increases, thus limiting the capacity of the cell.
It is mentioned that the causes of fracture of the .beta."-alumina solid electrolyte used in sodium-sulphur cells may be any one of (i) impurities in the liquid sodium, (ii) a current concentration above a certain critical level, and (iii) mechanical shock.
U.S. Pat. No. 4,029,858 proposes a partial solution to the problem of mixing of the reactants on fracture of the electrolyte, by providing a tube which contacts the liquid sodium, within the solid electrolyte tube. The outlet for sodium is at the bottom of the tube and liquid sodium moves to contact the inner wall of the electrolyte tube by a wicking action in the space between the storage tube and the electrolyte. However, in this arrangement, fracture of the solid electrolyte may lead to continuous slow engagement of the cell reactants.